1. Field of the Invention
The present invention relates to the field of MOSFET transconductance amplifiers, and automatic gain control circuits using MOSFET amplifiers.
2. Prior Art
Various transconductance amplifier circuits are well known in the prior art. Those disclosed herein, however, have various advantages in that they are realized in CMOS, have all n-channel active devices providing the amplification for faster response, have a wide output voltage swing, provide good linearity, and have expandability through additional current mirroring of the output.
In addition, automatic gain control circuits are also well known. In an automatic gain control circuit, as illustrated in FIG. 1, it is desirable to provide a constant change in the amplitude y of the signal y with a change in the gain control voltage V.sub.c, independent of the amplitude x of the input signal x. Since the gain G is the only variable element in the automatic gain control loop, this fixes the time response (dynamics) of the automatic gain control loop independent of the amplitude of the input signal, making loop stabilization easier and providing repeatable automatic gain control performance independent of the average input signal amplitude (within the automatic gain control range). In equation form, this provides: EQU G=f(V.sub.c)
(Gain is a function of the gain control voltage V.sub.c) EQU y=G*x=f(V.sub.c)*x
If the change in the amplitude y of the signal y with a change in the gain control voltage V.sub.c is to be independent of the amplitude x of the input signal x, then: ##EQU1##
Note that x is independent of V.sub.c, so that: ##EQU2##
Solving for the desired variation of G with V.sub.c : ##EQU3## where: k=a constant
Since y=G*x, and G=f(V.sub.c): ##EQU4##
Assuming y is at least approximately equal to the reference voltage V.sub.ref : ##EQU5## where: C=an arbitrary constant, and k'=k/V.sub.ref
Thus in an automatic gain control circuit, it is commonly desired to have a variable gain amplifier having an exponential variation in gain with respect to the gain control signal V.sub.c, Bipolar devices have a good exponential characteristic, and therefore bipolar automatic gain control circuits are commonly used. CMOS devices, however, do not have a good exponential characteristic, and therefore the realization of an automatic gain control circuit with the desired gain control response in CMOS is a much more difficult problem. The present invention transconductance amplifiers in conjunction with active CMOS load devices well approximate the desired gain control response while maintaining the advantages of the present invention transconductance amplifiers hereinbefore set forth.